1. Technical Field
The disclosed embodiments generally relate to the field of sintered diamond cutting and forming tools and more particularly to such diamond tools having extremely fine microstructures imparting improved tool properties, machinability, and an ability to impart improved surface finish to workpiece materials.
2. Description of the Related Art
Polycrystalline diamond (PCD) is used extensively in industrial applications including metal cutting, wire drawing, drilling, and as wear parts. As defined herein, PCD is a two phase polycrystalline diamond product in which the diamond crystals are sintered together to form a continuous diamond lattice. This lattice, the majority phase, comprises interparticle diamond-to-diamond bonds without interposed, non-diamond, bonding phases. A volume of residual catalyst metal, the minor phase, may be disposed in interstices between diamond crystals.
PCD production methods were first discovered in the 1960's and are well described in patent literature. U.S. Pat. Nos. 3,831,428; 4,063,909; 5,488,268, the disclosures of each of which are incorporated herein by reference, describe high pressure/high temperature (HP/HT) methods that produce cutting tools, wire drawing dies, and earth boring drilling cutters with resistance to abrasive and chemical wear. Because PCD exhibits more uniform mechanical properties than single crystal diamond and is available in larger sizes than single crystal diamond, PCD offers substantial design advantages over natural or synthetic single crystal diamond.
However, PCD as currently produced, does not provide extremely smooth cut, drawn or otherwise formed workpiece surfaces. Single crystal diamond, while expensive, anisotropic, and limited in size, remains the preferred tool material for single point turning of optical materials or drawing of highly finished, fine wire. Mechanical failure, from limited strength and impact resistance, of PCD tools is also common.
Conventional PCD is formed by infiltrating synthetic diamond grains with a suitable solvent catalyst material under processing conditions of extremely high pressure/high temperature, where the solvent catalyst material promotes desired intercrystalline diamond-to-diamond bonding between the grains, thereby forming a PCD structure. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired. Solvent catalyst materials typically used for forming conventional PCD include metals from Group VIII of the Periodic table, with cobalt (Co) being the most common.
Available PCD components have parts having diamond grain sizes after HP/HT sintering (“as-sintered”) of 1 μm to 100 μm. Finer, uniform, as-sintered diamond grain sizes, for example, of about 0.1 μm to about 1.0 μm (referred to as “submicron”) have proven challenging to produce commercially using the PCD manufacturing process described above. Submicron diamond particles are difficult to produce, and have proven difficult to handle during blending and mixing due to their high surface area's ability to attract and retain contaminants that affect the sintering process and product properties.
Submicron diamond particles have low packing densities that cause problems during loading of shielding enclosures and HP/HT processing. The very fine pores between the submicron diamond grains in the initial diamond particle mass are difficult to uniformly penetrate with catalyst metal, leading to incomplete bonding and sintering between diamond particles. It is almost always observed that the high surface area of submicron diamond powders causes the diamond solution-reprecipitation process to occur non-uniformly. This leads to non-uniform detrimental diamond grain growth and other complications that make the production of larger parts unfeasible when final diamond grain sizes less than 1 micron are attempted.
Prior attempts to produce submicron monolithic PCD have not yielded product having any substantial uniformity, either as (i) a monolithic, free-standing body, or (ii) PCD attached to a substrate, known as supported PCD. PCD, as used herein, refers to a sintered PCD body that is comprised of a continuous diamond matrix, diamond to diamond bonds, with or without catalyst metal. PCD is generally a two-phase material (diamond and catalyst), and may contain a minority third phase interposed between diamond grains, such as bonding carbides, nitrides, or borides.
Due to the difficulty of catalyst penetrating a bed of very fine, submicron diamond particles, the dispersion of the catalyst tends to become non-uniform. This affects the uniformity of the sintering process resulting in poorly sintered zones. Internal stresses that arise due to the composite two-phase nature of the body also distribute non-uniformly throughout the PCD and this results in cracking.
U.S. Pat. No. 4,303,442 to Hara et al., the disclosure of which is incorporated herein by reference, describes a method to sinter diamond materials for a cutting tool or wire die in which the grain size of the diamond is less than 1 μm. Hara et al. discusses the benefits of submicron grain structure in providing high dimensional precision and superb surface finish on workpieces. In order for Hara et al. to produce useful sintered diamond tools, it was necessary to add a third phase of one or more carbides, nitrides, and borides of IVB, VB, VIB group metals (otherwise known as International Union of Pure and Applied Chemistry (IUPAC) Group 4, Group 5, and Group 6 elements, respectively) together with an iron group catalyst metal to the submicron diamond particles.
Additionally, Hara et al. teaches the difficulty of producing submicron PCD. Example 1 of Hara et al. shows that submicron diamond powders with less than 5% of the bonding additions undergo grain growth to over 300 μm diameters during HP/HT sintering. These non-uniform materials were not hard enough to be useful as cutting tools.
There remains a need to produce a monolithic PCD material with uniform, as-sintered, diamond grains sizes below 1 μm. Surprisingly, applicants have found a method, utilizing a cobalt oxalate dihydrate as the source catalyst metal compound, to achieve several of the advantages of submicron PCD.
The disclosure contained herein describes attempts to address one or more of the problems described above.